System and method of preventing a surge condition in a vane-type compressor

ABSTRACT

A method of preventing compressor surge in an air conditioning system comprises: 1) determining the refrigerant flow rate at the compressor; 2) determining the input pressure and output pressure at the compressor; 3) determining the pressure ratio (output pressure/input pressure) at the compressor; 4) defining a surge limit based upon the pressure ratio and the refrigerant flow rate; 5) processing system operational inputs including motor speeds and air conditioning pressures; and 6) sending control signals to the air conditioning system to control the refrigerant flow rate and the pressure ratio in a manner to prevent compressor operation at the defined surge limit and to maximize compressor efficiency during transitional periods. An air conditioning system is also provided, as well as an article of manufacture for use with the system for preventing compressor surge.

RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/693,589, filed Aug. 1, 1996, abandoned, which is herebyincorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to centrifugal compressors and, moreparticularly, to a system and method of preventing compressor surge in aturbine-type refrigerant compressor with fixed vanes used in an airconditioning system.

BACKGROUND ART

"Surge" is an unstable impeller condition with potentially destructiveconsequences in a centrifugal compressor. As an example, if a typicalturbine engine has one occurrence of surge, the engine is removed fromservice, overhauled, and the impellers replaced.

A centrifugal compressor with fixed vanes operating at: 1) a fixed RPM;2) a fixed pressure ratio P_(O) \P_(I) (output pressure/input pressure);and 3) a fixed vapor mass flow rate, can go into surge with a suddenchange in evaporator load, the evaporator load being directly related tothe refrigerant flow rate. This phenomenon occurs when the evaporatorblower speed is reduced or when the evaporator inlet air enthalpy leveldrops quickly when changing from outside air to recirculated air. Thephenomenon is illustrated in FIG. 1, a chart illustrating generalcompressor performance characteristics with the vertical axisrepresenting pressure ratio (P_(O) /P_(I)) and the horizontal axisrepresenting refrigerant flow rate (M).

Point A in FIG. 1 represents a compressor operating at refrigerant flowrate M₁, pressure flow rate PR₁ and a speed of N=100% (of ratedcompressor RPM) Assume an ambient temperature of T₁ =100° F., an ambienthumidity level of H₁ =40% relative humidity, and an enthalpy level of E₁=42.7 BTU/lb. If the evaporator inlet air enthalpy decreases from E₁ toE₂ (defined by P₂ H₂ =75° F., 48% relative humidity; enthalpy of 27.9BTU/hr. from a psychometric chart) by changing from outside air torecirculated air, the compressor will go into surge by shifting frompoint A to point B where the refrigerant mass flow rate is reduced fromM₁ to M₂, where M₂ is the new flow rate required to satisfy theevaporator. In this example of a surge condition, if the compressormaintains a constant pressure ratio PR₁, the compressor speed is reducedfrom N=100 to N=86%. This causes the compressor to run under theconditions illustrated at point B in FIG. 1, in which case thecompressor is running at the surge line, and exceeding surge marginsSL_(norm) and SL_(max) at flow rate M₂.

Another surge condition would exist if the compressor begins at theconditions indicated at point A, and the compressor speed is maintainedconstant at N=100% and the pressure ratio is increased until thecompressor exceeds the surge limits and the surge line. In this case,the mass flow value associated with the N=100% compressor RPM line issignificantly greater than the desired M₂ flow rate at the surge line.This point is restricted by surge margin limit SL_(max) at point A'. Therefrigerant flow rate at A' is higher than the desired M₂ flow rate.

"Surge margins" are the difference between the compressor surge line andan actual acceptable operating condition at the same flow rate. Thecompressor pressure ratio and the compressor speed are reduced at thesame flow rate but with a surge margin reduced for a constant compressorspeed line. FIG. 1 illustrates the surge line and two surge margin lines(SL_(norm), SL_(max)) The greater the difference, the less chance thereis of encountering surge during transient conditions. The surge marginsare defined as follows: a) SL_(norm) is the normal surge limit and is aslow response curve; and b) SL_(max) is the fast response surge limitand is optional. The slow response surge margin values are based on flowrate change values with time constants and magnitudes substantially lessthan the fast change values. The actual magnitude of the surge marginline relative to the surge line is dependent on the individual system.The SL_(norm) surge margin is used during stabilized system operationwhere there are small changes in inlet enthalpy occurring over arelatively long period of time. The SL_(max) surge margin is enactedduring sudden large shifts in evaporator capacity over a very shortperiod of time, e.g., changing the evaporator blower speed or changingfrom outside air to recirculated air. The reason for the SL_(max) surgeline is to provide additional surge protection during large, shortduration system load changes.

Traditionally, turbine-based machines utilize a complex mechanicallyregulated vapor bypass which controls vapor flow through the compressorin order to permit such transitions to occur without going into surge.This results in a complex system of valves and ports that increase thesize, weight, and cost of a centrifugal machine.

Another means of controlling surge in centrifugal compressors isdescribed in Kountz et al., U.S. Pat. No. 4,546,618. This patentdescribes the use of variable inlet vanes, commonly calledpre-rotational vanes. These vanes require a driver motor, complexmechanical hardware, and pivotable turning vanes that must operate inhigh gas flow areas over a wide range of temperatures with a high degreeof accuracy. The device is microprocessor-controlled, but provides onlyan iterative process for avoiding surge, and does not providepredetermined paths for operating at optimized efficiency.

It is desirable to provide a surge control system which does not requiresuch complex vapor bypass controls or mechanically variable vanes forpreventing surge.

SUMMARY OF THE INVENTION

The present invention overcomes the above-referenced shortcomings ofprior art surge control systems by providing a system which prevents acompressor from operating either at surge or beyond the surge limitmargins by providing an electronic means of controlling the refrigerantmass flow by varying the compressor pressure ratio and the compressorspeed relative to the compressor surge line and relative to selectedsurge margins. The system includes: 1) a motor driven centrifugalcompressor with a fixed vane impeller; 2) a condenser; 3) an evaporator;4) a means of transferring heat from the condenser and evaporator; 5) ameans of expanding the refrigerant; 6) a means of storing and drying therefrigerant; 7) a microprocessor control interfacing with all key systemparameters; and 8) software to control the processor.

More specifically, the present invention provides a method of preventingcompressor surge in an air conditioning system, including: a compressorhaving an impeller with fixed vanes (or non-moving pre-rotationalvanes), the compressor operating at a pressure ratio (P_(O) /P_(I))defined by an output pressure (P_(O)) divided by an input pressure(P_(I)) and having a refrigerant flow rate (M), and operating at acompressor motor speed; a condenser in fluid communication with thecompressor, the condenser having a condenser fan or fans; an expansiondevice in fluid communication with the condenser; and an evaporator influid communication with the expansion device. The method comprises: 1)determining the refrigerant flow rate (M); 2) determining the inputpressure (P_(i)) and output pressure (P_(O)); 3) determining thepressure ratio (P_(O) /P_(I)); 4) defining a surge limit (SL) based uponthe pressure ratio (P_(O) /P_(I)) and the refrigerant flow rate (M); and5) sending control signals to adjust the compressor motor speed andcondenser fan speed to control the refrigerant flow rate (M) and thepressure ratio to prevent compressor operation beyond the defined surgelimit.

Another aspect of the present invention provides an air conditioningsystem, comprising a compressor having a compressor fan; a condenser influid communication with the compressor, the condenser having acondenser fan; an expansion device in fluid communication with thecondenser; an evaporator in fluid communication with the expansiondevice and including an evaporator fan; an expansion device; and aprocessor in electrical communication with the condenser fan, theevaporator fan, the expansion device, and the compressor. The processoris operative to limit compressor operation to predefined limits toprevent compressor surge and to define control paths during refrigerantflow rate changes.

The present invention also provides an article of manufacture used todirect a computer or other like programmable apparatus to controloperation of an air conditioning system, as described above. The articleof manufacture comprises a computer readable storage medium and acomputer program represented as computer readable data on the computerreadable storage medium, the computer program directing the computer toperform the steps described above.

Accordingly, an object of the invention is to provide an electronicmeans of preventing surge in a centrifugal compressor withoutmechanically variable vanes.

A further object is to provide an electronic means of preventing surgein a manner which eliminates additional hardware typically required forsurge prevention, and provides several layers of programmable surgemargin protection with differing response times.

Another object is to provide an electronic means of preventing surge ina centrifugal compressor with fixed vanes in a manner which providesmultiple control schemes for refrigerant flow rate changes, and providesa means of maintaining maximum system efficiency during the refrigerantflow rate change transition period.

The above objects and other objects, features, and advantages of thepresent invention are readily apparent from the following detaileddescription of the best modes for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a graphical representation of general compressorperformance characteristics with a vertical axis representing pressureratio, and a horizontal axis representing refrigerant flow rate;

FIG. 2 shows a schematic illustration of a surge control system for usein a refrigeration system in accordance with the present invention; and

FIG. 3 shows a schematic flow diagram of a control logic scheme inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention, as described with reference to FIGS. 1 and 2,provides a system and method of preventing a centrifugal compressor fromoperating either in surge or beyond the defined surge limit margins.

The air conditioning system shown in FIG. 2 utilizes the presentinvention. The air conditioning system 10 includes a compressor 12 withone or more impellers with fixed inlet vanes, or having rotational vaneswhich are not moved. Alternatively, the present invention may be usedwith compressor designs with vapor management systems not includingturning vanes or refrigerant straightening vanes. The compressorreceives refrigerant vapor under low pressure and compresses it to ahigh pressure, high temperature vapor. The high pressure, hightemperature vapor then enters the condenser 14 where heat is removed andthe vapor, as it cools, becomes a high pressure liquid refrigerant. Theliquid line 16 then carries the compressed liquid to the dryer 18 andthen to the expansion valve 20. As the refrigerant passes through theexpansion valve 20, the refrigerant's pressure is lowered. The lowpressure refrigerant then enters the evaporator 22 where it begins toboil and is changed into the vapor state by absorbing heat from the warmair passing over the evaporator 22.

From the evaporator 22, low pressure vapor may then pass through anaccumulator (not shown) en route to the compressor 12, thus completingthe closed loop system. Of course, an accumulator or an accumulator witha drier (not used with a high side receiver/dryer) could be used. Amicroprocessor control unit 24 is provided with software 26 foroperating the air conditioning system 10 in a manner to preventoperation of the compressor 12 under surge conditions.

The microprocessor or electronic control 24 incorporates algorithms viathe software 26 for interaction with the refrigeration system 10,specifically the evaporator fan 28, condenser fan 30 and compressor 12.This control limits the compressor operation to a specific surge marginline, thereby preventing the compressor from operating in surge. Themicroprocessor may also receive pressure and temperature readings fromvarious locations in the system. As shown in FIG. 2, the processorreceives inputs from thermal and pressure sensors, and calculates thereal-time condition and then, based on the magnitude and rate of theload change, makes incremental adjustments in condenser fan speed and incompressor speed.

Returning to FIG. 1, by way of example, we choose a compressor operatingat point A, and refrigerant flow rate M₁, a steady state condition. Theevaporator load is then reduced instantaneously, which occurs when theevaporator blower speed is reduced, thus requiring a reduction in therefrigerant flow rate to M₂. The microprocessor 24, using algorithmswritten for the system, reduces the compressor speed from 100% to 83%.Simultaneously, the condenser fan speed is adjusted to provide theappropriate pressure ratio, in this case PR₂. The compressor speed andpressure ratio are controlled by following one of the followingpaths: 1) A-A"-C; 2) A-A'-A"-C; 3) A-C (linear); or 4) A-C (non-linear).

FIG. 3 illustrates the control logic 40 in accordance with the presentinvention for preventing surge and operating at optimized efficiency.All inputs are read on a continuing real-time basis, as indicated at 42.The inlet and outlet pressures 44,46 of the compressor are read and thepressure ratio is calculated at 48. The compressor inlet temperature 50,compressor speed 52, and pressures 44,46 are input into a flow equationto determine the flow rate at 54. By knowing the refrigerant pressureand temperature, the state of the gas is known and a density is defined.A look-up table, determined empirically as described below, defines aflow rate as a function of these parameters, including compressor speed.The preferred embodiment is to use an equation for the look-up tablederived from the empirical data. This eliminates the need for derivingevery possible combination of compressor speed and refrigerant gastemperature and pressure.

Once the pressure ratio and refrigerant flow rate are known, thereal-time point is identified and compared to the surge line SL_(norm)at 56. If the compressor operating point is to the right of SL_(norm),the microprocessor then compares the current operating point to SL_(max)at 58.

If the current operating point is to the left of SL_(norm), themicroprocessor determines that the compressor is operating in surge andadjusts the compressor speed and condenser fan speed at 62 and 64 toeliminate the surge condition. This event could happen if the evaporatorblower speed were suddenly reduced. The control algorithm 60, in itspreferred embodiment, would select a path that maximizes the time spentwithin the peak adiabatic efficiency island by adjusting compressormotor speed 62 and condenser fan speed 64.

If the compressor is operating to the right of SL_(max), the algorithmthen returns to the start of the logic loop "read all inputs" 42, andcontinues through the control loop. If the compressor operating point isto the left of the SL_(max) line, the algorithm compares the currentoperating point to the SL_(norm) line to determine if a surge conditionexists.

The microprocessor surge control algorithm 60 monitors and controlsthese events on a millisecond basis in real-time. The algorithm, in itspreferred embodiment, also employs a response time variable so that ifthe compressor speed or pressure ratio changes faster than apredetermined rate, the control limit SL_(max) is used rather thanSL_(norm). The equations are of the form, AccelC=(New CRPM-OldCRPM)/time where AccelC is the acceleration or change in compressor RPMper unit of time and AccelP=(New PR-Old PR) /time where AccelP is therate of change in pressure ratio per unit of time. The algorithmreceives compressor speed and pressure ratio values and calculates therate of change of acceleration continuously. These control values can bedetermined empirically or by using engineering judgment and are includedin the look-up tables in the microprocessor for both SL_(norm) controland SL_(max) control as the control references.

The first control scheme is to vary the compressor speed from N=100% toN=90% while maintaining pressure ratio PR₁, until point A", the SL_(max)point, is reached. At this point, the compressor speed and the pressureratio are reduced simultaneously from A" to C along the SL_(max) surgelimit line.

The second control scheme is to maintain the compressor speed at N=100%and vary the pressure ratio along the constant speed line N=100% whilethe flow rate changes. The control path becomes A-A'-A"-C. At surgelimit A', defined as the maximum allowable pressure ratio at speedN=100% and PR_(max), the microprocessor 24 enables an algorithm thatmakes the following instantaneous adjustments: 1) the condenser fanspeed is adjusted to reduce the pressure ratio from PR_(max) to PR₂ ;and 2) the compressor speed is reduced from N=100% to N=83%. When thisis accomplished, the compressor is operating under the conditionsillustrated by point C in FIG. 1.

A third control scheme is for the compressor to go directly from A to Cfollowing a straight line that intersects both points. This control pathruns the compressor at its peak adiabatic efficiency for most of thechange, and also maintains a substantial cushion from the surge limitlines for most of the transition.

A fourth, and preferred, control scheme is for the compressor to go fromA to C following either a linear or a non-linear path to stay at peakadiabatic efficiency and minimum input power until reaching the desiredoperating point. This control path maximizes compressor operation at itspeak adiabatic efficiency and also maintains a substantial cushion fromthe surge limit lines for most of the transition. This path could be anS curve or a Z shape, or any other linear/non-linear combination,depending on the compressor and system operational characteristics.

Under control scheme 4, the microprocessor control algorithmcontinuously defaults as a function of system capability to match thespecified flow path. As an example, if it is not possible to achieve asimultaneous pressure ratio, flow rate, and compressor speed usingcontrol scheme 4, the microprocessor would continuously default untilthe desired operating point is reached so that it may reject controlschemes 4, 3, and 2 before selecting control scheme 1. Fuzzy logic andadaptive learning may also be integrated into the selection algorithmsto further optimize the control process.

These surge control adjustments may follow the surge limit curves orthey may be controlled with an additional surge margin during rapidtransient capacity changes. They may follow linear or non-linear paths.Typically, these algorithms are developed using empirical data generatedduring controlled laboratory testing. The speed and pressure ratiocontrol events can occur either simultaneously or independently. Theseevents occur in a step function, and the magnitude of each of theseevents is variable. Typical adjustment values may range from 0.5% to 5%,depending on the desired rate of change of the refrigerant mass flowrate. The rate of control is typically in the microsecond range. Fuzzylogic may be used to accelerate the rate of control changes based onevaluating the blower speed change and/or the inlet airtemperature/enthalpy change to reduce the response time and maintainacceptable surge margins. The compressor speed and pressure ratio arecontinuously varied until the refrigerant mass flow rate is stable atthe lower flow rate. This stability function is defined as Am/time wherethe time constant would typically be in the 5-20 second range. ##EQU1##

These control schemes are valid for both small reductions in evaporatorload that occur over a long period of time, i.e. as the weather changesor the temperature drops when the sun sets, etc., as well as substantialchanges in evaporator load such as encountered with switching fromoutside air to recirculated air or reducing the blower speed from highblower to low blower.

The method comprises the following steps:

1. Determining the refrigerant flow rate (M);

2. Determining the input pressure (P_(I)) and output pressure (P_(O));

3. Determining the pressure ratio (P_(O) /P_(I));

4. Defining a surge limit (SL) based upon the pressure ratio (P_(O)/P_(I)) and the refrigerant flow rate (M);

5. Defining a control path for the refrigerant flow rate and pressureratio to prevent compressor operation at the surge limit; and

6. Sending control signals to the air conditioning system to operate thesystem in accordance with the defined control path.

The step of determining the refrigerant flow rate M is determinedempirically using standard laboratory mapping techniques to determinethe flow rate as a function of compressor speed, compressor dischargepressure, and compressor inlet pressure. The ratio of these pressures isnormally used for plotting purposes. FIG. 1 illustrates a typicalempirical compressor map showing flow rate versus compressor pressureratio.

Centrifugal compressor empirical mapping is performed using acalorimeter. A calorimeter typically consists of a means of rejectingthe compressor and the condenser heat and a means of rejecting theevaporator heat while operating a compressor and monitoring and/orrecording system temperatures and pressures for both the refrigerant andthe air side heat transfer, i.e., the air flow volume and change in airtemperature caused when air flows over the heat exchangers. Acalorimeter normally measures compressor inlet and outlet pressures,compressor inlet and outlet temperatures, evaporator inlet and outlettemperature and pressure, condenser inlet and outlet temperature andpressure, and includes a means of determining a refrigerant flow volumethrough the compressor. A condenser with a fan or fans with variableflow control is an example of a means of transferring heat to an ambientenvironment. An evaporator with one or more fans with variable air flowis an example of a means of transferring heat from the evaporator coilto a control volume. Variable speed fans are used to allow precisecontrol of the heat rejection rate. These air moving devices arecalibrated using various types of orifices so that the exact air flowrate can be determined using a manometer. The air flow is normallycorrected to standard conditions to eliminate any error caused bychanges in air density due to differing atmospheric conditions such astemperature or humidity. Refrigerant control valves can be used to setrefrigerant temperatures so that testing may be run under standardconditions for comparative purposes. The compressor speed is alsomonitored and controlled.

The compressor mapping illustrated in FIG. 1 is determined by installinga centrifugal compressor on a calorimeter and operating it at a varietyof inlet and exit pressures and at a variety of compressor speeds. Eachpoint on the pressure ratio versus mass flow map is coincident with aspecific operating condition, i.e, for a given compressor inlet and exitpressure and compressor speed, there is a specific flow rate as well asa corresponding efficiency and heat rejection rate or capacity, measuredin BTU/HR.

The step of defining a surge limit includes, alternatively, defining anormal surge limit (SL_(norm)) suitable for limiting compressoroperation during relatively small evaporator load changes; or defining amaximum surge limit (SL_(max)) suitable for limiting compressoroperation during relatively large evaporator load changes.

The step of defining a control path consists of the following options:

1. Maintaining a constant compressor pressure ratio (P_(O) /P_(I));

2. Varying the compressor speed while the compressor pressure ratio(P_(O) /P_(I)) remains constant in a manner to define a control path;

3. Maintaining a constant compressor speed and varying the pressureratio and the refrigerant flow rate in a manner to define a controlpath;

4. Varying the compressor speed, pressure ratio and refrigerant flowrate in a manner to define a control path;

5. Varying the compressor speed, pressure ratio and refrigerant flowrate in a linear fashion to define a control path resulting in peakcompressor adiabatic efficiency throughout the flow rate change; and

6. Varying the compressor speed, pressure ratio, and refrigerant flowrate in a non-linear fashion to define a control path maximizingtransition time at peak compressor adiabatic efficiency whilemaintaining peak compressor adiabatic efficiency throughout the flowrate change.

The step of maintaining a constant compressor pressure ratio isaccomplished by the microprocessor which monitors all key systemfunctions as shown in FIG. 2. The condenser fan speed is adjusted tomaintain a constant pressure ratio as the compressor speed is reducedfrom A to A" shown in FIG. 1, as an example.

As described above, a calorimeter is used to map compressor performanceas a function of pressure ratio, refrigerant flow rate, and compressorspeed. The pressure ratio can be varied while maintaining a constantflow rate by varying the condenser fan speed and compressor speed for agiven evaporator load. The evaporator load is measured at the evaporatorbased upon flow rate, temperature of inlet air, and humidity level.Decreasing the condenser fan speed will cause an increase in compressoroutlet pressure and increasing the fan speed will decrease thecompressor outlet pressure at a given evaporator load, at a constantevaporator inlet pressure, and for a given condenser air inlettemperature.

A horizontal line can be drawn starting at the surge limit in FIG. 1 andextending to the right (increasing mass flow rate). This line isdetermined empirically by using a calorimeter to map the compressorrequirements as a function of increasing refrigerant flow rate. This isa straightforward process for those skilled in this field. These values,which include compressor inlet and outlet pressures and compressorspeed, are then plotted versus the dependent variable which is therefrigerant mass flow rate. Each point along this straight linerepresents a unique flow rate and a specific heat rejection capacity.Because of the nature of a centrifugal compressor, the efficiency canremain constant over a range of refrigerant flow rates.

Conversely, a vertical line may be drawn that represents a constantrefrigerant mass flow rate. This line will have a range of possiblepressure ratios starting at the surge limit and extending down(decreasing pressure ratio) that represent a variety of operatingefficiencies and pressure ratios for a given refrigerant flow rate.

The control path may also include a combination of vertical andhorizontal components, such as that shown by event A'-A".

A non-linear control path for controlling surge and maximizingefficiency combines the horizontal and the vertical control schemes inan exponential matter. An example of a non-linear control path isillustrated by line A-A' shown in FIG. 1.

Accordingly, three parameters are varied (i.e., compressor speed,pressure ratio, and refrigerant flow) to define a control path. Thecompressor flow rate is a function of the compressor speed and thepressure ratio, i.e., the compressor flow rate is a dependent variableand changes as a function of compressor speed and pressure ratio.Pressure ratio is dependent on condenser fan speed.

As described previously, the compressor flow rate is sensed in thecalorimeter for establishing the FIG. 1 map by measuring the volume flowof the condensed liquid refrigerant as it exits the condenser by usingflow meters readily available that are designed for this purpose. Thisis a straightforward method for determining the refrigerant flow rateand is a generally accepted laboratory practice. This value can also beverified through a variety of thermodynamic and mechanical means,including air side and refrigerant side condenser heat rejection, airside and refrigerant side evaporator heat rejection, compressor volumeflow rate, and the state of the compressor inlet gas.

The flow rate can be determined using the inlet temperature, the inletpressure, and the compressor speed. These values are measured using thecalorimeter. The flow relationship between the inlet and outlet pressureand the compressor speed is recorded and is programmed into themicroprocessor where it serves as a look-up table. In the presentmethod, the microprocessor continuously monitors in real-time the inletand outlet pressure and the compressor speed. The pressure ratio iscontrolled by changing the condenser air flow volume by varying thecondenser fan speed for a given compressor speed and system load. Theoperating efficiency ("N" in FIG. 1), because it is also a function ofthese same variables, is also, by definition, empirically known.

While the best modes for carrying out the invention have been describedin detail, those familiar with the art to which this invention relateswill recognize various alternative designs and embodiments forpracticing the invention within the scope of the appended claims.

What is claimed is:
 1. A method of preventing compressor surge in an airconditioning system, including a centrifugal compressor having animpeller with fixed inlet vanes, the compressor operating at a pressureratio (P_(O) /P_(I)) defined by an output pressure (P_(O)) divided by aninput pressure (P_(I)) and having a refrigerant flow rate (M), andoperating at a compressor motor speed; a condenser in fluidcommunication with the compressor, the condenser having a condenser fan;an expansion device in fluid communication with the condenser; and anevaporator in fluid communication with the expansion device andincluding an evaporator fan; the method comprising:determining therefrigerant flow rate (M); determining the input pressure (P_(I)) andoutput pressure (P_(O)); determining the pressure ratio (P_(O) /P_(I));defining a surge limit (SL) based upon the pressure ratio (P_(O) /P_(I))and the refrigerant flow rate (M) and sending control signals to adjustthe compressor motor speed and condenser fan speed to control therefrigerant flow rate (M) and the pressure ratio to prevent compressoroperation at the defined surge limit.
 2. The method of claim 1, furthercomprising the steps of:defining a control path for the refrigerant flowrate and pressure ratio to prevent compressor operation at the surgelimit; and sending control signals to the compressor and condenser fanto operate the system in accordance with the defined control path. 3.The method of claim 2, wherein said step of defining a surge limitcomprises defining a normal surge limit (SL_(norm)) suitable forlimiting compressor operation during relatively small evaporator loadchanges.
 4. The method of claim 2, wherein said step of defining a surgelimit comprises defining a maximum surge limit (SL_(max)) suitable forlimiting compressor operation during relatively large evaporator loadchanges.
 5. The method of claim 2, wherein said step of defining acontrol path further comprises:maintaining a constant compressorpressure ratio (P_(O) /P_(I)); and varying the compressor speed whilethe compressor pressure ratio (P_(O) /P_(I)) remains constant in amanner to define a control path.
 6. The method of claim 2, wherein saidstep of defining a control path comprises:maintaining a constantcompressor speed; and varying the pressure ratio and the refrigerantflow rate in a manner to define a control path, said varying step beingachieved by said sending of control signals to adjust the condenser fanspeed.
 7. The method of claim 2, wherein said step of defining a controlpath comprises:varying the compressor speed, pressure ratio andrefrigerant flow rate in a manner to define a control path, said varyingstep being achieved by said sending of control signals to adjust thecompressor motor speed and condenser fan speed.
 8. The method of claim2, wherein said step of defining a control path comprises:varying thecompressor speed, pressure ratio and refrigerant flow rate in a linearfashion to define a control path resulting in peak compressor adiabaticefficiency throughout the flow rate change, said varying step beingachieved by said sending of control signals to adjust the compressormotor speed and condenser fan speed.
 9. The method of claim 2, whereinsaid step of defining a control path comprises:varying the compressorspeed, pressure ratio, and refrigerant flow rate in a non-linear fashionto define a control path maximizing transition time at peak compressoradiabatic efficiency while maintaining peak compressor adiabaticefficiency throughout the flow rate change, said varying step beingachieved by said sending of control signals to adjust the compressormotor speed and condenser fan speed.
 10. An air conditioning system,comprising:a centrifugal compressor having an impeller with fixed vanes,said compressor operating at a pressure ratio (P_(O) /P_(I)) defined byan output pressure (P_(O)) divided by an input pressure (P_(I)) andhaving a refrigerant flow rate (M), and operating at a compressor motorspeed; a condenser in fluid communication with said compressor, saidcondenser having a condenser fan operating at a condenser fan speed; anexpansion device in fluid communication with said condenser; anevaporator in fluid communication with said expansion device; and aprocessor in electrical communication with said compressor, saidexpansion device, said condenser fan and said impeller, said processorbeing operative to limit compressor operation to predefined limits bysending control signals to adjust the compressor motor speed andcondenser fan speed to prevent compressor surge and to define controlpaths during refrigerant flow rate changes.
 11. The air conditioningsystem of claim 10, wherein the processor is further operativeto:determine the refrigerant flow rate; determine the input pressure(P_(I)) and output pressure (P_(O)); determine the pressure ratio (P_(O)/P_(I)) define a surge limit (SL) based upon the pressure ratio (P_(O)/P_(I)) and the refrigerant flow rate (M); and send control signals tothe compressor and condenser fan to control the refrigerant flow rate(M) and the pressure ratio to prevent compressor operation at thedefined surge limit.
 12. The air conditioning system of claim 11,wherein the processor is further operative to:define a control path forthe refrigerant flow rate and pressure ratio to prevent compressoroperation at the surge limit; and send control signals to the compressorand condenser fan to operate the system in accordance with the definedcontrol path.
 13. The air conditioning system of claim 12, wherein theprocessor is further operative to define a normal surge limit(SL_(norm)) suitable for limiting compressor operation during relativelysmall evaporator load changes.
 14. The air conditioning system of claim12, wherein the processor is further operative to define a surge limit(SL_(max)) suitable for limiting compressor operation during relativelylarge evaporator load changes.
 15. The air conditioning system of claim12, wherein the processor is further operative to:maintain a constantcompressor pressure ratio (P_(O) /P_(I)); and vary the compressor speedwhile the compressor pressure ratio (P_(O) /P_(I)) remains constant in amanner to define a control path.
 16. The air conditioning system ofclaim 12, wherein the processor is further operative to:maintain aconstant compressor speed; and vary the pressure ratio and therefrigerant flow rate by sending said control signals to adjust thecondenser fan speed in a manner to define a control path.
 17. The airconditioning system of claim 12, wherein the processor is furtheroperative to vary the compressor speed, pressure ratio and refrigerantflow rate in a manner to define a control path by sending said controlsignals to adjust the compressor motor speed and condenser fan speed.18. The air conditioning system of claim 12, wherein the processor isfurther operative to vary the compressor speed, pressure ratio andrefrigerant flow rate by sending said control signals to adjust thecompressor motor speed and condenser fan speed to define a linearcontrol path resulting in peak compressor adiabatic efficiencythroughout the flow rate change.
 19. The air conditioning system ofclaim 12, wherein the processor is further operative to vary thecompressor speed, pressure ratio, and refrigerant flow rate by sendingsaid control signals to adjust the compressor motor speed and condenserfan speed to define a non-linear control path maximizing transition timeat peak compressor adiabatic efficiency while maintaining peakcompressor adiabatic efficiency throughout the flow rate change.
 20. Anarticle of manufacture used to direct a computer or other likeprogrammable apparatus to control operation of an air conditioningsystem, including a compressor having an impeller with fixed vanes, saidcompressor operating at a pressure ratio (P_(O) /P_(I)) defined by anoutput pressure (P_(O)) divided by an input pressure (P_(I)) and havinga refrigerant flow rate (M), and operating at a compressor motor speed;a condenser in fluid communication with said compressor, said condenserhaving a condenser fan; an expansion device in communication with saidcondenser; and an evaporator in fluid communication with said expansiondevice; the article of manufacture comprising:a computer-readablestorage medium; and a computer program represented as computer-readabledata on the computer-readable storage medium, the computer programdirecting the computer to perform the steps of:determining therefrigerant flow rate (M); determining the input pressure (P_(I)) andoutput pressure (P_(O)); determining the pressure ratio (P_(O) /P_(I));defining a surge limit (SL) based upon the pressure ratio (P_(O) /P_(I))and the refrigerant flow rate (M); and sending control signals to adjustthe compressor motor speed and condenser fan speed to control therefrigerant flow rate (M) and the pressure ratio to prevent compressoroperation at the defined surge limit.
 21. The article of manufacture ofclaim 20, wherein the computer program directs the computer to performthe further steps of:defining a control path for the refrigerant flowrate and pressure ratio to prevent compressor operation at the surgelimit; and sending control signals to the compressor and condenser fanto operate the system in accordance with the defined control path. 22.The article of manufacture of claim 21, wherein the step of defining asurge limit comprises defining a normal surge limit (SL_(norm)) suitablefor limiting compressor operation during relatively small evaporatorload changes.
 23. The article of manufacture of claim 21, wherein thestep of defining a surge limit comprises defining a maximum surge limit(SL_(max)) suitable for limiting compressor operation during relativelylarge evaporator load changes.
 24. The article of manufacture of claim21, wherein the step of defining a control path furthercomprises:maintaining a constant compressor pressure ratio (P_(O)/P_(I)); and varying the compressor speed while the compressor pressureratio (P_(O) /P_(I)) remains constant in a manner to define a controlpath.
 25. The article of manufacture of claim 21, wherein the step ofdefining a control path comprises:maintaining a constant compressorspeed; and varying the pressure ratio and the refrigerant flow rate bysending said control signals to adjust the condenser fan speed in amanner to define a control path.
 26. The article of manufacture of claim21, wherein said step of defining a control path comprises:varying thecompressor speed, pressure ratio and refrigerant flow rate by sendingsaid control signals to adjust the compressor motor speed and condenserfan speed in a manner to define a control path.
 27. The article ofmanufacture of claim 21, wherein said step of defining a control pathcomprises:varying the compressor speed, pressure ratio and refrigerantflow rate by sending said control signals to adjust the compressor motorspeed and condenser fan speed to define a linear control path resultingin peak compressor adiabatic efficiency throughout the flow rate change.28. The article of manufacture of claim 21, wherein said step ofdefining a control path comprises:varying the compressor speed, pressureratio, and refrigerant flow rate by sending said control signals toadjust the compressor motor speed and condenser fan speed to define anon-linear control path maximizing transition time at peak compressoradiabatic efficiency while maintaining peak compressor adiabaticefficiency throughout the flow rate change.